A group of national security experts and American foreign policy luminaries sent a letter on Monday to the primary co-sponsors of an Iran sanctions bill asking them to reconsider the measure, saying it jeopardizes the ongoing negotiations with Tehran over its nuclear program and could bring the United States closer to war with Iran.

After various avenues to put forth Iran sanctions measures recently failed, Sens. Robert Menendez (D-NJ) and Mark Kirk (R-IL) introduced the stand-alone bill late last month. Nearly 50 senators — mostly Republicans — have signed on as co-sponsors, but the chairs of 10 Senate committees recently wrote to Senate Majority Harry Reid (D-NV) slamming the bill and asking him not to move forward with it. The White House has said it will veto the bill if it passes.

In the letter sent to Menendez and Kirk on Monday, the group of experts — which includes former U.S. Ambassador to Iraq and Afghanistan Ryan Crocker, former U.S. Ambassador to Israel and Egypt Daniel Kurtzer, William H. Luers, the former Ambassador to Venezuela and Czechoslovakia, and Jessica Tuchman Mathews, the President of the Carnegie Endowment for International Peace — say their bill “will threaten the prospects for success in the current negotiations and thus present us and our friends with a stark choice — military action or living with a nuclear Iran.” Crocker et al note that attacking Iran would not preventit from developing nuclear weapons and would most likely give the Iranians the justification to decide to seek them — “the very thing the U.S. hopes to prevent,” they write.

The interim agreement reached between Iran, the U.S. and its international partners in Geneva last November significantly reined in Iran’s nuclear program in exchange for modest sanctions relief (most polls show that Americans support this first step deal). But Obama administration officials and Iran experts believe that passing new sanctions on Iran now — even those with a delayed trigger as the Menendez-Kirk bill mandates — would violate the terms of Geneva’s Joint Plan of Action, thus jeopardizing any final deal with Iran.

The letter’s signatories share that concern and address the argument that lawmakers often make when pushing more sanctions now: the threat of harsh penalties will strengthen the U.S.’s negotiating position. “To the contrary,” the letter says, “Iranian leaders are more likely to see such Congressional action as a violation of the spirit and perhaps the letter of the Joint Plan of Action of November 24, 2013, and to harden rather than soften their negotiating position.” They note that Iranian parliamentarians have already introduced a measure to enrich nearly weapons-grade uranium in response to the Menendez-Kirk bill. “This kind of tit-for-tat spiral threatens to undermine any possibility of curtailing Iran’s nuclear program,” they write.

“Should the U.S. Congress decide it must unilaterally seek to add even more burdens now on this complicated and critical process, it is unlikely that the goals of our negotiations can be achieved,” they write, warning that “our other negotiating partners (UK, France, Germany, Russia, and China) would be displeased and would conclude that the US is no longer proceeding in good faith in accord with the Joint Plan of Action. This bill could lead to an unraveling of the sanctions regime that the U.S. and its partners have so patiently built.”

CQ Roll Call reported last week that Reid “still has not publicly signaled his intentions on a floor vote” on the Kirk-Menendez bill.

“The bill had 47 co-sponsors signed up before Christmas and we expect at least a dozen more to sign up in the first couple of days back in session,” a Senate aide said via e-mail to CQ. “Once there are 60 co-sponsors, meaning the bill can clear a cloture motion, it will be difficult for Harry Reid to delay a vote on the bill; if it gets to a veto-proof majority of co-sponsors, it will be nearly impossible.”

“If we appear to be going off on our own tangent and do whatever we want we will potentially lose their support for the sanctions themselves because we don’t just enforce them by ourselves, we need their help,” Kerry said, referring the U.K, France, China, Russia and Germany. “I don’t want to threaten the unity that we currently have with respect to this approach particularly when it doesn’t cost us a thing to go through this process knowing that we could put sanctions in place additionally in a week and we would be there with you seeking to do it. I don’t want to give the Iranians public excuse to flout the agreement.”

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